Oxide superconductors are known to form by the solid state reaction of a stoichiometric mixture of metal oxides, carbonates, nitrates or oxalates and the like at temperatures above 500.degree. C. In addition to the desired oxide superconductive phase, the reaction produces intermediate oxide phases. Consumption of the intermediate phases and complete conversion to the desired oxide superconductor requires long reaction times and multiple mixing and annealing steps. The intermediate oxide may be either non-superconductive or superconductive. For example, in the reaction to form Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x (BSCCO-2223), Bi.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.x (BSCCO-2212) is formed as an intermediate phase.
Alternatively, the desired oxide superconductor can also be formed from the reaction of a mechanical mixture of an intermediate oxide and secondary non-superconductive oxides. For example, the oxide superconductor Y.sub.1 Ba.sub.2 Cu.sub.4 O.sub.x (YBCO-124) can be prepared from a mixture of the oxide superconductor Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.x (YBCO-123) and copper oxide, see Jin et al., Appl. Phys. Lett. 56, p.1287 (1990). Although the use of an intermediate superconductive oxide provides a better mixing of some components of the system before reaction, the equi-axed granular structure of the secondary non-superconductive oxides still causes a significant degree of non-homogeneity. As well, long reaction times and multiple mixing and annealing steps are still required.
Performance of the oxide superconductor, and of articles formed from such oxide superconductors, is related to the degree of orientation or "texture" of the superconductive oxide grains. Texture is induced by the preferential growth of the grains in the ab plane within a constrained volume during thermal heat treatment, and by deformation processes which align anisotropic (e.g., plate-like) oxide superconductor grains in the plane of minimum compressive stress. The equi-axed or granular structure of the secondary non-superconductive oxides disrupts the planar alignment of the plate-like oxide superconductor grains in two ways. The non-superconductive oxides prevent efficient deformation-induced texture by impeding the alignment of the oxide superconductor grains and by preventing full densification of the material. For instance, note the equi-axed or granular secondary non-superconductive oxides 10 in FIG. 1. The equi-axed grains 10 also interrupt the preferential grain growth of thermally induced texturing. The relatively low surface area of the oxide grains necessitates a long reaction time to "react away" the disruptive secondary non-superconductive oxide particles.
Therefore, it is highly desired to eliminate large, equi-axed secondary metal salts, including non-superconductive oxides, in the formation and texturing of an oxide superconductor. Use of finer particle-size metal salts may somewhat improve the rate of formation of the oxide superconductor. However, such use cannot prevent the spatially non-uniform dispersion of metal salt particles, which will adversely affect both texture and formation rates.
It is therefore an object of the present invention to overcome the above limitations of the prior art and to provide an oxide precursor powder which can rapidly and efficiently form the desired oxide superconductor.
It is yet another object of the present invention to provide an oxide precursor powder that is readily deformable, so that grains of the oxide superconductor can be aligned.
It is yet another object of the present invention to provide a method for producing the above oxide precursor powder.